Coarse frequency synchronisation in multicarrier systems

ABSTRACT

A method for generating a signal having a frame structure, each frame of the frame structure comprising at least one useful symbol, a guard interval associated to the at least one useful symbol and a reference symbol, comprises the steps of performing an amplitude modulation of a bit sequence such that the envelope of the amplitude modulated bit sequence defines a reference pattern of the reference symbol and inserting the amplitude modulated bit sequence into said signal as said reference symbol. A method for frame synchronization of a signal having such a frame structure comprises the steps of receiving the signal, down-converting the received signal, performing an amplitude-demodulation of the down-converted signal in order to generate an envelope, correlating the envelope with a predetermined reference pattern in order to detect a signal reference pattern of the reference symbol in the signal, and performing the frame synchronization based on the detection of the signal reference pattern.

FIELD OF THE INVENTION

The present invention relates to methods and apparatus for generating asignal having a frame structure, wherein each frame of the framestructure is composed of useful symbols, a guard interval associated toeach useful symbol and one reference symbol. In addition, the presentinvention relates to methods and apparatus for frame synchronization ofsignals having the above structure.

The present invention is particularly useful in a MCM transmissionsystem (MCM=Multi-carrier modulation) using an orthogonal frequencydivision multiplexing (OFDM) for digital broadcasting.

BACKGROUND OF THE INVENTION

In a MCM (OFDM) transmission system the binary information isrepresented in the form of a complex spectrum, i.e. a distinct number ofcomplex subcarrier symbols in the frequency domain. In the modulator abitstream is represented by a sequence of spectra. Using an inverseFourier-transform (IFFT) a MCM time domain signal is produced from thissequence of spectra.

In case of a transmission of this described MCM signal via a multipathchannel with memory, intersymbol interference (ISI) occurs due tomultipath dispersion. To avoid ISI a guard interval of fixed length isadded between adjacent MCM symbols in time. The guard interval is chosenas cyclic prefix. This means that the last part of a time domain MCMsymbol is placed in front of the symbol to get a periodic extension. Ifthe fixed length of the chosen guard interval is greater than themaximum multipath delay, ISI will not occur.

In the receiver the information which is in the frequency and timedomain (MCM) has to be recovered from the MCM time domain signal. Thisis performed in two steps. Firstly, optimally locating the FFT window,thus eliminating the guard interval in front of each MCM time domainsymbol. Secondly, performing a Fourier Transform of the sequence ofuseful time samples thus obtained.

As a result a sequence of spectral symbols is thus recovered. Each ofthe symbols contains a distinct number of information carryingsubcarrier symbols. Out of these, the information bits are recoveredusing the inverse process of the modulator.

Performing the above described method, the following problem occurs inthe receiver. The exact position of the guard interval and hence theposition of the original useful parts of the time domain MCM symbols isgenerally unknown. Extraction of the guard interval and the subsequentFFT-transform of the resulting useful part of the time signal is notpossible without additional information. To provide this additionalinformation, a known (single carrier) sequence in the form of a (timedomain) reference symbol is inserted into the time signal. With theknowledge about the positions of the reference symbols in the receivedsignal, the exact positions of the guard intervals and thus theinteresting information carrying time samples are known.

The periodical insertion of the reference symbol results in a framestructure of the MCM signal. This frame structure of a MCM signal isshown in FIG. 1. One frame of the MCM signal is composed of a pluralityof MCM symbols 10. Each MCM symbol 10 is formed by an useful symbol 12and a guard interval 14 associated therewith. As shown in FIG. 1, eachframe comprises one reference symbol 16.

A functioning synchronization in the receiver, i.e. frame, frequency,phase, guard interval synchronization is necessary for the subsequentMCM demodulation. Consequently, the first and most important task of thebase band processing in the receiver is to find and synchronize to thereference symbol.

DESCRIPTION OF THE PRIOR ART

Most prior art methods for frame synchronization have been developed forsingle carrier transmission over the AWGN channel (AWGN=Additive WhiteGaussian Noise). These prior art methods based on correlation are,without major changes, not applicable for transmission over multipathfading channels with large frequency offsets or MCM transmission systemsthat use, for example, an orthogonal frequency division multiplexing.

For MCM transmission systems particular frame synchronization methodshave been developed.

Warner, W. D., Leung C.: OFDM/FM Frame Synchronization for Mobile RadioData Communication, IEEE Trans. On Vehicular Technology, vol. VT-42,August 1993, pp. 302 to 313, teaches the insertion of reference symbolsin the form of tones in parallel with the data into the MCM symbol. Thereference symbols occupy several carriers of the MCM signal. In thereceiver, the synchronization carriers are extracted in the frequencydomain, after a FFT transform (FFT=fast Fourier transform) using acorrelation detector. In the presence of large frequency offsets, thisalgorithm becomes very complex because several correlators must beimplemented in parallel.

A further prior art technique is to insert a periodic reference symbolinto the modulated MCM signal. This reference symbol is a CAZAC sequence(CAZAC=Constant Amplitude Zero Autocorrelation). Such techniques aretaught by: Classen, F., Meyr, H.: Synchronization algorithms for an OFDMsystem Vehic. Technology Conference, 1997; Schmidl, T. M., Cox, D. C.:Low-Overhead, Low-Complexity [Burst] Synchronization for OFDMTransmission, Proc. IEEE Int. Conf. on Commun., 1996. In such systems,the receiver's processor looks for a periodic repetition. For thesealgorithms coarse frequency synchronization has to be achieved prior toor at least simultaneously with frame synchronization.

Van de Beek, J, Sandell, M., Isaksson, M, Börjesson, P.: Low-ComplexFrame Synchronization in OFDM Systems, Proc. of the ICUPC, 1995, avoidthe insertion of additional reference symbols or pilot carriers and useinstead the periodicity in the MCM signal which is inherent in the guardinterval and the associated cyclical extension. This method is suitableonly for slowly varying fading channels and small frequency offsets.

U.S. Pat. No. 5,191,576 relates to a method for the diffusion of digitaldata designed to be received notably by mobile receivers moving in anurban environment. In this method, the header of each frame of abroadcast signal having a frame structure has a first emptysynchronization symbol and a second unmodulated wobbled signal forming atwo-stage analog synchronization system. The recovery of thesynchronization signal is achieved in an analog way, without priorextraction of a clock signal at the binary level.

EP 0631406 A relates to data signals, COFDM signals, for example, and tomethods and apparatus for diffusing said signals. The COFDM signalscomprises a sequence of symbols, each symbol having an useful portionand a guard interval. Two symbols of a COFDM signal are provided assynchronization symbols. One of the two symbols is a zero symbol,whereas the other thereof is a synchronization symbol which is formed byan unmodulated multiplex of the carrier frequencies having a constantenvelope. Beside the two symbols as synchronization symbols, it istaught in EP 0631406 A to modulate the pilot frequency of the datasignal with a reference signal which carries the synchronizationinformation. This reference signal modulated on the pilot frequency ofthe data signal can be used by a MABLR demodulator.

WO 98/00946 A relates to a system for a timing and frequencysynchronization of OFDM signals. Two OFDM training symbols are used toobtain full synchronization in less than two data frames. The OFDMtraining symbols are placed into the OFDM signal, preferably at leastonce every frame. The first OFDM training symbol is produced bymodulating the even-numbered OFDM sub-carriers whereas the odd-numberedOFDM sub-carriers are suppressed. Thus, in accordance with WO 98/00946A, the first OFDM training symbol is produced by modulating theeven-numbered carriers of this symbol with a first predetermined PNsequence.

Moose: “A technique for orthogonal frequency division multiplexingfrequency offset correction”, IEEE TRANSACTIONS ON COMMUNICATIONS, Vo.42, No. 10, October 1994, pages 2908 to 2914, teaches methods forcorrecting frequency offsets in OFDM digital communications. The methodsinvolve repetition of a data symbol and comparison of the phases of eachof the carriers between the successive symbols. The phase shift of eachof the carriers between the repeated symbols is due to the frequencyoffset since the modulation phase values are not changed in the repeatedsymbols.

Keller; Hanzo: “Orthogonal frequency division multiplex synchronizationtechniques for wireless local area networks”, IEEE INTERNATIONALSYMPOSIUM ON PERSONAL, INDOOR AND MOBILE RADIO COMMUNICATIONS, Oct. 15,1996, pages 963 to 967, teach frequency acquisition, frequency tracking,symbol synchronization and frame synchronization techniques. Regardingthe frame synchronization, it is taught to use a reference symbol whichconsists of repetitive copies of a synchronization pattern ofpseudo-random samples. The frame synchronization is achieved byautocorrelation techniques using the periodic synchronization segmentssuch that for the synchronization algorithms proposed no a prioriknowledge of the synchronization sequences is required.

The methods for frame synchronization available up to date requireeither prior achieved frequency synchronization or become very complexwhen the signal in the receiver is corrupted by a large frequencyoffset.

If there is a frequency offset in the receiver, as can easily be thecase when a receiver is powered-on and the frequency synchronizationloop is not yet locked, problems will occur. When performing a simplecorrelation there will only be noise at the output of the correlator,i.e. no maximum can be found if the frequency offset exceeds a certainbound. The size of the frequency offset depends on the length (time) ofthe correlation to be performed, i.e. the longer it takes, the smallerthe allowed frequency offset becomes. In general, frequency offsetincreases implementation complexity.

Frequency offsets occur after power-on or later due to frequencydeviation of the oscillators used for down-conversion to baseband.Typical accuracies for the frequency of a free running local oscillator(LO) are at ±50 ppm of the carrier frequency. With a carrier frequencyin S-band (e.g. 2.34 GHz) there will be a maximum LO frequency deviationof above 100 kHz (117.25 kHz). A deviation of this magnitude puts highdemands on the above methods.

In the case of multipath impaired transmission channel, a correlationmethod yields several correlation maxima in addition to the distinctmaximum for an AWGN channel. The best possible frame header position,i.e. the reference symbol, has to be selected to cope with this numberof maxima. In multipath channels, frame synchronization methods withcorrelations can not be used without major changes. Moreover, it is notpossible to use data demodulated from the MCM system, because thedemodulation is based on the knowledge of the position of the guardinterval and the useful part of the MCM symbol.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method and anapparatus for generating a signal having a frame structure that allow aframe synchronization after the signals have been transmitted even inthe case of a carrier frequency offset or in the case of a transmissionvia a multipath fading channel.

It is a further object of the present invention to provide a method andan apparatus for frame synchronization of a signal having a framestructure even in the case of a carrier frequency offset.

In accordance with a first aspect, the present invention provides amethod for generating a signal having a frame structure, each frame ofthe frame structure comprising at least one useful symbol, a guardinterval associated to the at least one useful symbol and a referencesymbol, the method comprising the steps of performing an amplitudemodulation of a bit sequence, the envelope of the amplitude modulatedbit sequence defining the reference pattern of the reference symbol andinserting the amplitude modulated bit sequence into said signal as saidreference symbol.

In accordance with a second aspect, the present invention provides amethod for generating a multi-carrier modulated signal having a framestructure, each frame of the frame structure comprising at least oneuseful symbol, a guard interval associated to the at least one usefulsymbol and a reference symbol, the method comprising the steps of:

-   -   providing a bitstream;    -   mapping bits of the bitstream to carriers in order to provide a        sequence of spectra;    -   performing an inverse Fourier transform in order to provide        multi-carrier modulated symbols;    -   associating a guard interval to each multi-carrier modulated        symbol;    -   generating the reference symbol by performing an amplitude        modulation of a bit sequence, the envelope of the amplitude        modulated bit sequence defining the reference pattern of the        reference symbol;    -   associating the reference symbol to a predetermined number of        multi-carrier modulated symbols and associated guard intervals        in order to define the frame; and    -   inserting said amplitude modulated bit sequence into said signal        as said reference symbol.

In accordance with a third aspect, the present invention provides amethod for frame synchronization of a signal having a frame structure,each frame of the frame structure comprising at least one useful symbol,a guard interval associated with the at least one useful symbol and areference symbol, the method comprising the steps of:

-   -   receiving the signal;    -   down-converting the received signal;    -   performing an amplitude-demodulation of the down-converted        signal in order to generate an envelope;    -   correlating the envelope with a predetermined reference pattern        in order to detect the signal reference pattern of the reference        symbol in the signal; and    -   performing the frame synchronization based on the detection of        the signal reference pattern.

In accordance with a fourth aspect, the present invention provides amethod for frame synchronization of a multi-carrier modulated signalhaving frame structure, each frame of the frame structure comprising atleast one useful symbol, a guard interval associated to the at least oneuseful symbol and a reference symbol, the method comprising the stepsof:

-   -   receiving the multi-carrier modulated signal;    -   down-converting the received multi-carrier modulated signal;    -   performing an amplitude-demodulation of the down-converted        multi-carrier modulated signal in order to generate an envelope;    -   correlating the envelope with a predetermined reference pattern        in order to detect the signal reference pattern of the reference        symbol in the multi-carrier modulated signal;    -   performing the frame synchronization based on the detection of        the signal reference pattern;    -   extracting the reference symbol and the at least one guard        interval from the down-converted received multi-carrier        modulated signal based on the frame synchronization;    -   performing a Fourier transform in order to provide a sequence of        spectra from the at least one useful symbol;    -   de-mapping the sequence of spectra in order to provide a        bitstream.

In accordance with a fifth aspect, the present invention provides anapparatus for generating a signal having a frame structure, each frameof the frame structure comprising at least one useful symbol, a guardinterval associated to the at least one useful symbol and a referencesymbol, the apparatus comprising an amplitude modulator for performingan amplitude modulation of a bit sequence, the envelope of the amplitudemodulated bit sequence defining the reference pattern of the referencesymbol; and

-   means for inserting the amplitude modulated bit sequence into said    signal as said reference symbol.

In accordance with a sixth aspect, the present invention provides anapparatus for generating a multi-carrier modulated signal having a framestructure, each frame of the frame structure comprising at least oneuseful symbol, a guard interval associated to the at least one usefulsymbol and a reference symbol, the apparatus comprising:

-   -   means for providing a bitstream;    -   means for mapping bits of the bitstream to carriers in order to        provide a sequence of spectra;    -   means for performing an inverse Fourier transform in order to        provide multi-carrier modulated symbols;    -   means for associating a guard interval to each multi-carrier        modulated symbol;    -   means for generating the reference symbol by an amplitude        modulator for performing an amplitude modulation of a bit        sequence, the envelope of the amplitude modulated bit sequence        defining the reference pattern of the reference symbol;    -   means for associating the reference symbol to a predetermined        number of multi-carrier modulated symbols and associated guard        intervals in order to define the frame; and    -   means for inserting the amplitude modulated bit sequence into        said signal as said reference symbol.

In accordance with a seventh aspect, the present invention provides anapparatus for frame synchronization of a signal having a framestructure, each frame of the frame structure comprising at least oneuseful symbol, a guard interval associated to the at least one usefulsymbol and a reference symbol, the apparatus comprising:

-   -   receiving means for receiving the signal;    -   a down-converter for down-converting the received signal;    -   an amplitude-demodulator for performing an amplitude        demodulation of the down-converted signal in order to generate        an envelope;    -   a correlator for correlating the envelope with a predetermined        reference pattern in order to detect the signal reference        pattern of the reference symbol in the signal; and    -   means for performing the frame synchronization based on the        detection of the signal reference pattern.

In accordance with a eighth aspect, the present invention provides anapparatus for frame synchronization of a multi-carrier modulated signalhaving a frame structure, each frame of the frame structure comprisingat least one useful symbol, a guard interval associated to the at leastone useful symbol and a reference symbol, the apparatus comprising:

-   -   a receiver for receiving the multi-carrier modulated signal;    -   a down-converter for down-converting the received multi-carrier        modulated signal;    -   an amplitude-demodulator for performing an        amplitude-demodulation of the down-converted multi-carrier        modulated signal in order to generate an envelope;    -   a correlator for correlating the envelope with a predetermined        reference pattern in order to detect the signal reference        pattern of the reference symbol in the multi-carrier modulated        signal;    -   means for performing the frame synchronization based on the        detection of the signal reference pattern;    -   means for extracting the reference symbol and the at least one        guard interval from the down-converted received multi-carrier        modulated signal based on the frame synchronization in order to        generate the at least one useful symbol;    -   means for performing a Fourier transform in order to provide a        sequence of spectra from the at least one useful symbol; and    -   means for de-mapping the sequence of spectra in order to provide        a bitstream.

The present invention provides a novel structure of the reference symbolalong with a method to determine the position of the reference symboland thus the start of a frame in a signal having a frame structure asshown for example in FIG. 1.

The invention relates to a method for finding frame headersindependently of other synchronization information and thus forpositioning the FFT windows correctly. This includes the extraction of aguard interval. The method is based on the detection of a knownreference symbol of the frame header in the reception signal, e.g. inthe digital complex baseband. The new frame synchronization will beperformed as the first synchronization task.

Synchronization to the reference symbol, i.e. the frame header is thefirst step to initiate radio reception. The reference symbol isstructured to accomplish this. The information contained in thereference symbol must therefore be independent of other synchronizationparameters, e.g. frequency offset. For this reason, in accordance withthe present invention, the form of the reference symbol selected is anamplitude modulated sequence (AM sequence) in the complex baseband.Thus, the information contained in the reference symbol is only thatgiven in the amplitude and not that in the phase. Note that the phaseinformation will be corrupted by a possible frequency offset. Inpreferred embodiments of the present invention, the AM information isconstructed from a bit sequence with special features. The informationsequence is selected in a way which makes it easy and secure to find itin the time domain. A bit sequence with good autocorrelation propertiesis chosen. Good autocorrelation properties means a distinct correlationmaximum in a correlation signal which should be as white as possible.

A pseudo random bit sequence (PRBS) having good autocorrelationproperties meets the above requirements.

Using the envelope of the signal to carry bit information offersadditional flexibility. First it has to be decided which envelope valuesshould correspond to the binary values of 0 and 1. The parameters aremean amplitude and modulation rate. Attention should be paid toselecting the mean amplitude of the reference symbol (performance)identically to the mean amplitude of the rest of the frame. This is dueto the amplitude normalization (AGC; AGC=Automatic Gain Control)performed in the receiver. It is also possible to select the meanamplitude of the reference symbol higher than the mean signal amplitude,but then care has to be taken that the time constant of the AGC(1/sensitivity) is selected high enough to secure that the strong(boosted) signal of the reference symbol does not influence the AGCcontrol signal and thus attenuate the signal following the referencesymbol.

Another degree of freedom can be characterized as modulation degree d.This parameter is responsible for the information density of themodulating signal mod(t) formed out of the binary sequence bin(t) asfollows: mod(t)=bin(t/d). This modulation degree can be chosen as freeparameter fixed by an integer or real relation to the sampling rate. Itis appropriate to choose the modulation degree d as an integer valuebecause of the discrete values of the binary sequence:

d = 1: mod (m) = bin (m) d = 2: mod (m) = bin (m/2) for m even =bin_(—)int (m/2) for m odd d = 3: mod (m) = bin (m/3)for m = 0, ±3, ±6, ±9,   bin_(—)int (m/3) else

The signal values bin_(—)int(m/d) are computed from the binary sequencebin(m) by ideal interpolation (between the discrete integer values m)with the factor of d. This is similar to an ideal sampling rateexpansion (with sin(x)/x interpolation), but the sampling rate remains,only less bits of the binary sequence bin(m) correspond to the resultinginterpolated sequence mod(m). This parameter m indicates the discretetime.

With increasing m the modulating signal mod(t) is expanded in timerelative to the basic binary sequence, this results in a bandwidthcompression of the resulting AM spectrum with regard to the basic binarysequence. A time expansion by a factor 2 results in a bandwidthcompression by the same factor 2. In addition to the bandwidthcompression, a further advantage of a higher modulation degree d is areduced complexity of the search method in the receiver due to the factthat only each dth sample has a corresponding binary value. Choosing thefactor d=1 is not preferred since this would result in aliasing due todisregard of the sampling theorem. For this reason, in a preferredembodiment of the present invention d is chosen to be 2.

The choice of length and repetition rate of the reference symbol is, onthe one hand, dominated by the channel properties, e.g. the channel'scoherence time. On the other hand the choice depends on the receiverrequirements concerning mean time for initial synchronization and meantime for resynchronization after synchronization loss due to a channelfade.

In the receiver, the first step after the down-conversion of thereceived signal is to perform an amplitude-demodulation of thedown-converted signal in order to generate an envelope, i.e. in order todetermine the amplitude of the signal. This envelope is correlated witha replica reference pattern in order to detect the signal referencepattern of the reference symbol in the signal. In the case of a AWGNchannel, the result of this correlation will be a white noise signalwith zero mean value and with a clearly visible (positive) maximum. Inthe case of a multipath channel, several maxima will occur in thecorrelation signal computed by this correlation. In the former case, thelocation of the reference symbol is determined based on the signalmaximum, whereas in the latter case a weighting procedure is performedin order to find out the maximum corresponding to the location of thereference symbol.

Thus, the present invention shows how to find a reference symbol by adetection method which is simple. Furthermore, the present invention canbe used for one-carrier or multi-carrier systems. The present inventionis particularly useful in multi-carrier modulation systems using anorthogonal frequency division multiplexing, for example in the field ofdigital broadcasting. The synchronization methods according to thepresent invention are independent of other synchronization steps. Sincethe information needed for the synchronization is contained in theenvelope of the preamble, i.e. the reference symbol, the referencesymbol is independent of possible frequency offsets. Thus, a derivationof the correct down sampling timing and the correct positioning of theFFT window can be achieved. The reference symbol of the presentinvention can be detected even if the frequency synchronization loop isnot yet locked or even in the case of a carrier frequency offset. Theframe synchronization method in accordance with the present invention ispreferably performed prior to other and without knowledge of othersynchronization efforts.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, preferred embodiments of the present invention will beexplained in detail on the basis of the drawings enclosed, in which:

FIG. 1 shows a schematic view of a signal having a frame structure;

FIG. 2 shows a block diagram of a MCM system to which the presentinvention can be applied;

FIG. 3 shows a schematic block diagram of a frame and frequencysynchronization system in a MCM receiver;

FIG. 4 shows a schematic diagram of an apparatus for framesynchronization; and

FIG. 5 shows a typical channel impulse response of a single frequencynetwork in S-band.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although the present invention is explained mainly referring to a MCMsystem, it is obvious that the present invention can be used inconnection with different signal transmissions that are based ondifferent kinds of modulation.

FIG. 2 shows a MCM system overview on the basis of which the presentinvention will be described in detail. At 100 a MCM transmitter is shownthat substantially corresponds to a prior art MCM transmitter except forthe kind of the reference symbol being added to each frame of a MCMsignal. A description of such a MCM transmitter can be found, forexample, in William Y. Zou, Yiyan Wu, “COFDM: AN OVERVIEW”, IEEETransactions on Broadcasting, vol. 41, No. 1, March 1995.

A data source 102 provides a serial bitstream 104 to the MCMtransmitter. The incoming serial bitstream 104 is applied to abit-carrier mapper 106 which produces a sequence of spectra 108 from theincoming serial bitstream 104. An inverse fast Fourier transform (FFT)110 is performed on the sequence of spectra 108 in order to produce aMCM time domain signal 112. The MCM time domain signal forms the usefulMCM symbol of the MCM time signal. To avoid intersymbol interference(ISI) caused by multipath distortion, a unit 114 is provided forinserting a guard interval of fixed length between adjacent MCM symbolsin time. In accordance with a preferred embodiment of the presentinvention, the last part of the useful MCM symbol is used as the guardinterval by placing same in front of the useful symbol. The resultingMCM symbol is shown at 115 in FIG. 2 and corresponds to the MCM symbol10 depicted in FIG. 1. signal transmitted through the channel 122 isreceived at the receiver front end 132. The down-converted MCM signal issampled at the receiver front end 132 and is, in the preferredembodiment, provided to a fast running automatic gain control (timeconstant<MCM symbol duration) in order to eliminate fast channelfluctuations (channel coherence time=MCM symbol duration). The fast AGC162 is used in addition to the normally slow AGC in the signal path, inthe case of transmission over a multipath channel with long channelimpulse response and frequency selective fading. The fast AGC adjuststhe average amplitude range of the signal to the known average amplitudeof the reference symbol. The so processed symbol is provided to anamplitude determining unit 164.

The amplitude determining unit 164 can use the simple alpha_(max+)beta_(min−) method in order to calculate the amplitude of the signal.This method is described for example in Palachels A.: DSP-mP RoutineComputes Magnitude, EDN, Oct. 26, 1989; and Adams, W. T., and Bradley,J.: Magnitude Approximations for Microprocessor Implementation, IEEEMicro, Vol. 3, No. 5, October 1983.

The output signal of the amplitude determining unit 164 is applied to acorrelator 166. In the correlator 166, a cross correlation between theamplitude signal output from the amplitude determining unit 164 and aknown ideal amplitude information is computed. The known ideal amplitudeinformation is stored in the correlator. For both, the amplitude and theknown ideal amplitude information, their amplitudes are symmetrically tozero relative to their average amplitude.

In the ideal AWGN case, the result will be a white noise signal withzero mean value and with a clearly visible positive maximum. In thisideal AWGN case, the position of the single maximum is evaluated in amaximum position unit 172. On the basis of this evaluation, thereference symbol and the guard intervals are extracted from the MCMsignal in a combined reference symbol/guard extraction unit 136/138.Although these units are shown as a combined unit 136/138 in FIG. 4, itis clear that separate units can be provided. The MCM signal istransmitted from the RF front end 150 to the reference symbol/guardextraction unit 136/138 via a low pass filter 174.

In the case of time spreading encountered in a multipath channel,several maxima corresponding to the number of clusters in the channelimpulse response occur in the output signal of the correlator. Aschematic view of three such clusters located in a time window ofmaximum about 60 microseconds is shown in FIG. 5. Out of the severalmaxima caused by the time spreading encountered in a multipath channel,the best one has to be selected as the position of the frame header,i.e. the reference symbol. Therefore, a threshold unit 168 and aweighting unit 170 are provided between the correlator 166 and themaximum position unit 172. The threshold unit 168 is provided to removemaxima having an amplitude below a predetermined threshold. Theweighting unit 164 is provided in order to perform a weighting procedureon the remaining maxima such that the maximum corresponding to thereference symbol can be determined. An examplary weighting procedureperformed in the weighting unit 170 is as follows.

The first significant maximum is considered to be the best one. Theoutput signal of the correlator is observed from the first detectedmaximum onwards for the maximum length of the channel impulse responseand an amplitude weighting function is applied to the signal. Becausethe actual channel impulse response length is unknown, the followingfact can be remembered. During system design, the length of the channelimpulse response has to be investigated. In a MCM system, the guardinterval shall be equal or longer than the maximum expected channelimpulse response. For this reason, the part (interval with l_(I)samples, l_(I) corresponding to the maximum expected channel impulseresponse, i.e. the guard interval length) of the correlation outputsignal starting with the first maximum,I _(k0)(n)=r(k ₀ +n), 0≦n≦l _(I)−1  (Eq. 1)with k₀ being the position of the first maximum, will be examined tofind the best frame start position. The above signal part is weightedwith the function $\begin{matrix}{{W(n)} = 10^{{- \frac{weight\_ dB}{10}}\frac{n}{l_{I} - 1}}} & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$The position (n_(max)) of the maximum in the resulting signal interval$\begin{matrix}{{{I_{k,\;{weighted}}(n)} = {\left\lbrack {{r\left( {k_{0} + n} \right)}{W(n)}} \right\rbrack = \left\lbrack {{r\left( {k_{0} + n} \right)}10^{{- \frac{weight\_ dB}{10}}\frac{n}{l_{I} - 1}}} \right\rbrack}}{0 \leq n \leq {l_{I} - 1}}} & \left( {{Eq}.\mspace{14mu} 3} \right)\end{matrix}$will be chosen as best frame start position.

r(k) designates the output signal of the correlator (166) at the time k.The signal is present with a clock frequency which is determined by themultiplication: oversampling factor*subcarrier symbol frequency. Theparameter k designates the discrete time in sample clocks. This signalis windowed with information from the threshold unit 168. An intervalhaving the length of l_(I) values is extracted from the signal r(k). Thefirst value being written into the interval is the correlation startvalue at the time k₀, at which the output value r(k₀) exceeds thethreshold value of the threshold unit 168 for the first time. Theinterval with the windowed signal is designated by the term I(k₀). Theparameter n designates the relative time, i.e. position, of a valueinside the interval.

Using the described weighting operation, the earlier correlation maximaare more likely to be chosen as right frame start position. A latercoming maximum will only be chosen as frame start position, if the valueof the maximum is significantly higher than the earlier one. Thisoperation is applicable especially for MCM, because here it is better todetect the frame start positions some samples too early than somesamples too late. Positioning the frame start some samples too earlyleads to positioning the FFT window a little bit into the guardinterval, this contains information of the same MCM symbol and thereforeleads to little effects. If the frame start position is detected somesamples too late, then the FFT window includes some samples of thefollowing guard interval.

This leads to a more visible degradation, because the following guardinterval contains information of the following MCM symbol (ISI occurs).

It is important to know that the first visible correlation maximum afterreceiver power-on does not necessarily correspond to the first CIR(channel impulse response) cluster. It is possible that it iscorresponding to a later cluster, see FIG. 5. For this reason duringpower-on one should wait for a second frame start before startingdemodulation.

It is clear that amplitude determining methods different from thedescribed alpha_(max+) beta_(min−) method can be used. Forsimplification, it is possible to reduce the amplitude calculation to adetection as to whether the current amplitude is above or below theaverage amplitude. The output signal then consists of a −1/+1 sequencewhich will be correlated with a known bit sequence, also in −1/+1values. This correlation can easily be performed using a simpleintegrated circuit (IC).

In addition, an oversampling of the signal received at the RF front endcan be performed. For example, the received signal can be expressed withtwo times oversampling.

This oversampled signal is passed to a fast running AGC to eliminatefast channel fluctuations before the amplitude of the signal iscalculated. The amplitude information will be hard quantized. Valueslarger than the mean amplitude, mean amplitude is 1, will be expressedas +1, values smaller than the mean amplitude will be expressed as −1.This −1/+1 signal is passed to the correlator that performs a crosscorrelation between the quantized signal and the stored ideal amplitudevalues of the reference symbol:

-   amp_(—)sto(k)=2*bin(k/4),    -   if k=2(oversampling factor)*2(interpolation factor)* 1,2,3 . . .        92    -   (92 for 184 reference symbol and interpolation factor 2)-   amp_(—)sto(k)=0, else, k<=2(oversampling factor)* plary weighting    procedure performed in the weighting unit 170 is as follows.

The first significant maximum is considered to be the best one. Theoutput signal of the correlator is observed from the first detectedmaximum onwards for the maximum length of the channel impulse responseand an amplitude weighting function is applied to the signal. Becausethe actual channel impulse response length is unknown, the followingfact can be remembered. During system design, the length of the channelimpulse response has to be investigated. In a MCM system, the guardinterval shall be equal or longer than the maximum expected channelimpulse response. For this reason, the part (interval with lI samples,lI corresponding to the maximum expected channel impulse response, i.e.the guard interval length) of the correlation output signal startingwith the first maximum,I _(k0)(n)=r(k ₀ +n), 0# n # l_(I)−1  (Eq.1)with k₀ being the position of the first maximum, will be examined tofind the best frame start position. The above signal part is weightedwith the function $\begin{matrix}{{W(n)} = 10^{{- \frac{weight\_ dB}{10}}\frac{n}{l_{I} - 1}}} & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$(Eq.2)The position (n_(max)) of the maximum in the resulting signal interval$\begin{matrix}{{{I_{{k0},{weighted}}(n)} = {\left\lbrack {{r\left( {k_{0} + n} \right)}{W(n)}} \right\rbrack = \left\lbrack {{r\left( {k_{0} + n} \right)}10^{{- \frac{weight\_ dB}{10}}\frac{n}{l_{I} - 1}}} \right\rbrack}}0\mspace{14mu}\#\mspace{14mu} n\mspace{14mu}\#\mspace{14mu} l_{I}\; 1} & \left( {{Eq}.\mspace{14mu} 3} \right)\end{matrix}$(Eq.3)will be chosen as best frame start position.

r(k) designates the output signal of the correlator (166) at the time k.The signal is present with a clock frequency which is determined by themultiplication: oversampling factor*subcarrier symbol frequency. Theparameter k designates the discrete time in sample clocks. This signalis windowed with information from the threshold unit 168. An intervalhaving the length of l_(I) values is extracted from the signal r(k). Thefirst value being written into the interval is the correlation startvalue at the time k₀, at which the output value r(k₀) exceeds thethreshold value of the threshold unit 168 for the first time. Theinterval with the windowed signal is designated by the term I(k₀). Theparameter n designates the relative time, i.e. position, of a valueinside the interval.

Using the described weighting operation, the earlier correlation maximaare more likely to be chosen as right frame start position. A latercoming maximum will only be chosen as frame start position, if the valueof the maximum is significantly higher than the earlier one. Thisoperation is applicable especially for MCM, because here it is better todetect the frame start positions some samples too early than somesamples too late. Positioning the frame start some samples too earlyleads to positioning the FFT window a little bit into the guardinterval, this contains information of the same MCM symbol and thereforeleads to little effects. If the frame start position is detected somesamples too late, then the FFT window includes some samples of thefollowing guard interval. This leads to a more visible degradation,because the following guard interval contains information of thefollowing MCM symbol (ISI occurs).

It is important to know that the first visible correlation maximum afterreceiver power-on does not necessarily correspond to the first CIR(channel impulse response) cluster. It is possible that it iscorresponding to a later cluster, see FIG. 5. For this reason duringpower-on one should wait for a second frame start before startingdemodulation.

It is clear that amplitude determining methods different from thedescribed alpha₊ beta_(min−) method can be used. For simplification, itis possible to reduce the amplitude calculation to a detection as towhether the current amplitude is above or below the average amplitude.The output signal then consists of a −1/+1 sequence which will becorrelated with a known bit sequence, also in −1/+1 values. Thiscorrelation can easily be performed using a simple integrated circuit(IC).

In addition, an oversampling of the signal received at the RF front endcan be performed. For example, the received signal can be expressed withtwo times oversampling.

This oversampled signal is passed to a fast running AGC to eliminatefast channel fluctuations before the amplitude of the signal iscalculated. The amplitude information will be hard quantized. Valueslarger than the mean amplitude, mean amplitude is 1, will be expressedas +1, values smaller than the mean amplitude will be expressed as −1.This −1/+1 signal is passed to the correlator that performs a crosscorrelation between the quantized signal and the stored ideal amplitudevalues of the reference symbol:

-   amp_(—)sto(k)=2*bin(k/4),    -   if k=2(oversampling factor)*2(interpolation factor)* 1,2,3 . . .        92    -   (92 for 184 reference symbol and interpolation factor 2)-   amp_(—)sto(k)=0, else, k<=2(oversampling factor)    -   *2(interpolation factor)*92    -   (first part of amp_(—)sto=[0 0 0 −1 0 0 0 1 0 0 0 1 0 0 0 −1 0 .        . . ]).

With this algorithm a correlation maximum of 92 is achievable.

Again, the maxima in the correlator output signal correspond todifferent frame start positions due to different multipath clusters. Inthis signal with various maxima the best frame start position has to bechosen. This is done in the following steps: The output of thecorrelator is given to a threshold detection. If the signal first timeexceeds the threshold (a threshold of 50 has proved to be applicable)the best position search algorithm is initialized. The correlator outputsignal in the interval following the threshold exceeding value will beweighted with the weighting function, see above. The position of theresulting maximum in the weighted signal will be chosen as best framestart position. With the knowledge about the best frame start positionthe guard interval extraction and the following MCM demodulation will beperformed.

Some more efforts can be carried out to increase frame synchronizationaccuracy. These methods will be explained in the following.

A postprocessing of the frame start decision is performed in order a) toincrease the reliability of the frame synchronization; b) to secure thatno frame start position is disregarded; and c) to optimize the framestart position in case of varying CIR cluster positions.

Using information of other frame start positions. It is known that infront of each frame a reference symbol is inserted into the signal. Ifthe position of the currently detected frame start has changedsignificantly regarding the last detected frame start, demodulation ofthe two frames in total and completely independent from each other ispossible. It is also possible to buffer the last signal frame and toperform the required shift of the frame start position step by step withthe MCM symbols of the frame. This results in an interpolativepositioning of the single MCM symbols including simultaneousasynchronous guard interval extraction for the different MCM symbols.

Such an interpolative positioning of the FFT window is also possible ifone frame start position is missing, i.e. the frame start has not beendetected. If one frame start position is missing the guard intervalextraction can be performed the same way as in the frame before withoutlarge performance degradation. This is due to the normally only slowlyvarying CIR cluster positions, but only if the signal strength is goodenough. Stopping demodulation and waiting for the next detected framestart position is also imaginable but not desirable because of the longinterrupt.

What follows is an example of a reference symbol of 184 samples(subcarrier symbols) as provided by the inventive apparatus forgenerating a signal having a frame structure.

The underlying binary sequence of length 92 is:

bin = [0 1 1 0 1 1 0 1 0 1 1 0 1 0 1 0 0 0 1 1 1 0 0 0 0 0 0 0 0 1 1 0 11 1 1 1 0 0 0 1 1 1 0 0 0 0 0 0 0 1 1 1 0 1 1 1 0 0 1 1 0 1 1 1 0 1 1 01 0 1 0 1 1 0 1 1 0 1 1 0 1 0 0 0 0 1 0 1 1 0]The modulated binary sequence is:

i_(—)q = [0.5 1.5 1.5 0.5 1.5 1.5 0.5 1.5 0.5 1.5 1.5 0.5 1.5  0.5 1.50.5 0.5 0.5 1.5 1.5 1.5 0.5 0.5 0.5 0.5 0.5  0.5 0.5 0.5 1.5 1.5 0.5 1.51.5 1.5 1.5 1.5 0.5 0.5  0.5 1.5 1.5 1.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 1.51.5  1.5 0.5 1.5 1.5 1.5 0.5 0.5 1.5 1.5 0.5 1.5 1.5 1.5  0.5 1.5 1.50.5 1.5 0.5 1.5 0.5 1.5 1.5 0.5 1.5 1.5  0.5 1.5 0.5 1.5 0.5 0.5 0.5 0.51.5 0.5 1.5 1.5 0.5]This modulated binary sequence i_(—)q is interpolated in order toproduce an interpolated sequence i_(—)q_(—)int:

i_(—)q_(—)int = [0.5000 1.0635 1.5000 1.7195 1.5000 0.8706 0.5000 0.8571 1.5000 1.7917 1.5000 0.8108 0.5000 1.0392  1.5000 1.0392 0.50000.8108 1.5000 1.7984 1.5000  0.8108 0.5000 1.0460 1.5000 0.9997 0.50000.9603  1.5000 1.1424 0.5000 0.3831 0.5000 0.4293 0.5000  0.9997 1.50001.5769 1.5000 1.5769 1.5000 1.0065  0.5000 0.3899 0.5000 0.5325 0.50000.4931 0.5000  0.4999 0.5000 0.4931 0.5000 0.5325 0.5000 0.3967  0.50000.9603 1.5000 1.7522 1.5000 0.8571 0.5000  0.8965 1.5000 1.6422 1.50001.4669 1.5000 1.4737  1.5000 1.6096 1.5000 0.9929 0.5000 0.4226 0.5000 0.4226 0.5000 0.9997 1.5000 1.5769 1.5000 1.5769  1.5000 1.0065 0.50000.3899 0.5000 0.5325 0.5000  0.4931 0.5000 0.4931 0.5000 0.5325 0.50000.3899  0.5000 1.0065 1.5000 1.5701 1.5000 1.6096 1.5000  0.8965 0.50000.8965 1.5000 1.6096 1.5000 1.5633  1.5000 1.0392 0.5000 0.2867 0.50000.9929 1.5000  1.7454 1.5000 0.8571 0.5000 0.9033 1.5000 1.6028  1.50001.6028 1.5000 0.9033 0.5000 0.8503 1.5000  1.7917 1.5000 0.8108 0.50001.0460 1.5000 0.9929  0.5000 0.9929 1.5000 1.0460 0.5000 0.8108 1.5000 1.7917 1.5000 0.8571 0.5000 0.8571 1.5000 1.7849  1.5000 0.8571 0.50000.8571 1.5000 1.7917 1.5000  0.8176 0.5000 1.0065 1.5000 1.1424 0.50000.3436  0.5000 0.5788 0.5000 0.3436 0.5000 1.1424 1.5000  1.0065 0.83121.5000 1.7263 1.5000 1.0635 0.5000  0.0637]

amp _(—) int=i _(—) q _(—) int+j*i _(—) q _(—) int

amp_(—)int is the reference symbol inserted periodically into the signalafter the guard interval insertion.

As it is clear from the above specification, the present inventionprovides methods and apparatus for generating a signal having a framestructure and methods and apparatus for frame synchronization whenreceiving such signals which are superior when compared with prior artsystems. The frame synchronization algorithm in accordance with thepresent invention provides all of the properties shown in Table 1 incontrary to known frame synchronization procedures. Table 1 shows acomparison between the system in accordance with the present inventionusing an AM sequence as reference symbol and prior art systems (singlecarrier and MCM Eureka 147).

TABLE 1 Single carrier (e.g. QPSK like MCM Eureka MCM with AM WS) 147sequence Carrier offset no yes yes allowed Constant power yes no yesachieved at Rx input Coarse frequency no no yes offset estimationpossible Coarse channel yes no yes estimation possible (clusterestimation)

As can be seen from Table 1 different synchronization tasks andparameters can be derived using the frame synchronization with an AMsequence in accordance with the present invention. The framesynchronization procedure MCM Eureka 147 corresponds to the proceduredescribed in U.S. Pat. No. 5,191,576.

What is claimed is:
 1. A method for generating a signal having a framestructure, each frame of said frame structure comprising at least oneuseful symbol, a guard interval associated to said at least one usefulsymbol and a reference symbol, said method comprising the step ofperforming an amplitude modulation of a bit sequence, an envelope of theamplitude modulated bit sequence defining a reference pattern of saidreference symbol; and inserting, in time domain, the reference symbolinto said signal, wherein said reference symbol comprises a real partand an imaginary part, said real part and said imaginary part beingequal and being formed by said amplitude modulated bit sequence.
 2. Themethod according to claim 1, wherein said signal is an orthogonalfrequency division multiplexed signal.
 3. The method according to claim1, wherein said amplitude modulation is performed such that a meanamplitude of said reference symbol substantially corresponds to a meanamplitude of the remaining signal.
 4. The method according to claim 1,wherein said bit sequence is a pseudo random bit sequence having goodautocorrelation characteristics.
 5. The method according to claim 1,wherein a number of useful symbols in each frame is defined depending onchannel properties of a channel through which the signal or amulti-carrier modulated signal is transmitted.
 6. A method forgenerating a multi-carrier modulated signal having a frame structure,each frame of said frame structure comprising at least one usefulsymbol, a guard interval associated to said at least one useful symboland a reference symbol, said method comprising the steps of: providing abitstream; mapping bits of said bitstream to carriers in order toprovide a sequence of spectra; performing an inverse Fourier transformin order to provide multi-carrier modulated symbols; associating a guardinterval to each multi-carrier modulated symbol; generating saidreference symbol by performing an amplitude modulation of a bitsequence, an envelope of the amplitude modulated bit sequence defining areference pattern of said reference symbol; associating said referencesymbol to a predetermined number of multi-carrier modulated symbols andassociated guard intervals in order to define said frame; and inserting,in time domain, said reference symbol into said signal, wherein saidreference symbol comprises a real part and an imaginary part, said realpart and said imaginary part being equal and being formed by saidamplitude modulated bit sequence.
 7. The method according to claim 6,wherein said multi-carrier modulated signal is an orthogonal frequencydivision multiplex signal.
 8. The method according to claim 6, whereinsaid amplitude modulation is performed such that a mean amplitude ofsaid reference symbol substantially corresponds to a mean amplitude ofthe remaining multi-carrier modulated signal.
 9. A method for framesynchronization of a signal having a frame structure, each frame of saidframe structure comprising at least one useful symbol, a guard intervalassociated with said at least one useful symbol and a reference symbol,said reference symbol comprising a real part and an imaginary part, saidreal part and said imaginary part being equal and being formed by anamplitude modulated bit sequence, said method comprising the steps of:receiving said signal; down-converting said received signal; in timedomain, performing an amplitude-demodulation of said down-convertedsignal in order to generate an envelope; in time domain, correlatingsaid envelope with a predetermined reference pattern in order to detecta signal reference pattern of said reference symbol in said signal; andperforming said frame synchronization based on the detection of saidsignal reference pattern.
 10. The method according to claim 9, furthercomprising the step of performing a fast automatic gain control of saidreceived down-converted signal prior to the step of performing saidamplitude-demodulation.
 11. The method according to claim 9, wherein thestep of performing said amplitude-demodulation comprises the step ofcalculating an amplitude of said signal using the alpha_(max+)beta_(min−) method.
 12. The method according to claim 9, furthercomprising the steps of sampling respective amplitudes of said receiveddown-converted signal and comparing said sampled amplitudes with apredetermined threshold in order to generate a bit sequence in order toperform said amplitude demodulation.
 13. The method according to claim12, wherein the step of sampling respective amplitudes of said receiveddown-converted signal further comprises the step of performing anover-sampling of said received down-converted signal.
 14. The methodaccording to claim 9, further comprising the step of applying a resultof the frame synchronization for a frame in said signal to at least onesubsequent frame in said signal.
 15. The method according to claim 9,further comprising the step of detecting a location of said signalreference pattern based on an occurrence of a maximum of a correlationsignal when correlating said envelope with said predetermined referencepattern.
 16. The method according to claim 15, further comprising thesteps of: weighting a plurality of maxima of said correlation signalsuch that a maximum occurring first is weighted stronger than anysubsequently occurring maximum; and detecting said location of saidsignal reference pattern based on the greatest one of said weightedmaxima.
 17. The method according to claim 16, further comprising thestep of: disabling the step of performing said frame synchronization fora predetermined period of time after having switched-on a receiverperforming said method for frame synchronization.
 18. A method for framesynchronization of a multi-carrier modulated signal having framestructure, each frame of said frame structure comprising at least oneuseful symbol, a guard interval associated to said at least one usefulsymbol and a reference symbol, said reference symbol comprising a realpart and an imaginary part, said real part and said imaginary part beingequal and being formed by an amplitude modulated bit sequence, saidmethod comprising the steps of: receiving said multi-carrier modulatedsignal; down-converting said received multi-carrier modulated signal; intime domain, performing an amplitude-demodulation of said down-convertedmulti-carrier modulated signal in order to generate an envelope; in timedomain, correlating said envelope with a predetermined reference patternin order to detect a signal reference pattern of said reference symbolin said multi-carrier modulated signal; performing said framesynchronization based on the detection of said signal reference pattern;extracting said reference symbol and said at least one guard intervalfrom said down-converted received multi-carrier modulated signal basedon said frame synchronization; performing a Fourier transform in orderto provide a sequence of spectra from said at least one useful symbol;and de-mapping said sequence of spectra in order to provide a bitstream.19. The method according to claim 18, further comprising the step ofperforming a fast automatic gain control of said received down-convertedmulti-carrier modulated signal prior to the step of performing saidamplitude-demodulation.
 20. The method according to claim 18, whereinthe step of performing said amplitude-demodulation comprises the step ofcalculating an amplitude of said multi-carrier modulated signal usingthe alpha_(max+) beta_(min−) method.
 21. The method according to claim18, further comprising the steps of sampling respective amplitudes ofsaid received down-converted multi-carrier modulated signal andcomparing said sampled amplitudes with a predetermined threshold inorder to generate a bit sequence in order to perform said amplitudedemodulation.
 22. The method according to claim 21, wherein the step ofsampling respective amplitudes of said received down-convertedmulti-carrier modulated signal further comprises the step of performingan over-sampling of said received down-converted multi-carrier modulatedsignal.
 23. The method according to claim 18, further comprising thestep of applying a result of the frame synchronization for a frame insaid signal to at least one subsequent frame in said multi-carriermodulated signal.
 24. An apparatus for generating a signal having aframe structure, each frame of said frame structure comprising at leastone useful symbol, a guard interval associated to said at least oneuseful symbol and a reference symbol, said apparatus comprising: anamplitude modulator for performing an amplitude modulation of a bitsequence, an envelope of the amplitude modulated bit sequence defining areference pattern of said reference symbol; and means for inserting, intime domain, the reference symbol into said signal, wherein saidreference symbol comprises a real part and an imaginary part, said realpart and said imaginary part being equal and being formed by saidamplitude modulated bit sequence.
 25. The apparatus according to claim24, wherein said signal is an orthogonal frequency division multiplexedsignal.
 26. The apparatus according to claim 24, wherein a meanamplitude of said reference symbol substantially corresponds to a meanamplitude of the remaining signal.
 27. The apparatus according to claim24, comprising means for determining a number of useful symbols in eachframe depending on channel properties of a channel through which thesignal or a multi-carrier modulated signal is transmitted.
 28. Anapparatus for generating a multi-carrier modulated signal having a framestructure, each frame of said frame structure comprising at least oneuseful symbol, a guard interval associated to said at least one usefulsymbol and a reference symbol, said apparatus comprising: means forproviding a bitstream; means for mapping bits of said bitstream tocarriers in order to provide a sequence of spectra; means for performingan inverse Fourier transform in order to provide multi-carrier modulatedsymbols; means for associating a guard interval to each multi-carriermodulated symbol; means for generating said reference symbol comprisingan amplitude modulator for performing an amplitude modulation of a bitsequence, an envelope of the amplitude modulated bit sequence defining areference pattern of said reference symbol; means for associating saidreference symbol to a predetermined number of multi-carrier modulatedsymbols and associated guard intervals in order to define said frame;and means for inserting, in time domain, the reference symbol into saidsignal, wherein said reference symbol comprises a real part and animaginary part, said real part and said imaginary part being equal andbeing formed by said amplitude modulated bit sequence.
 29. The apparatusaccording to claim 28, wherein said multi-carrier modulated signal is anorthogonal frequency division multiplex signal.
 30. The apparatusaccording to claim 28, wherein said means for generating said referencesymbol performs the amplitude modulation such that a mean amplitude ofsaid reference symbol substantially corresponds to a mean amplitude ofthe remaining multi-carrier modulated signal.
 31. The apparatusaccording to claim 28, wherein said means for generating said referencesymbol generates a pseudo random bit sequence having goodautocorrelation characteristics as said bit sequence.
 32. An apparatusfor frame synchronization of a signal having a frame structure, eachframe of said frame structure comprising at least one useful symbol, aguard interval associated to said at least one useful symbol and areference symbol, said reference symbol comprising a real part and animaginary part, said real part and said imaginary part being equal andbeing formed by an amplitude modulated bit sequence, said apparatuscomprising: receiving means for receiving said signal; a down-converterfor down-converting said received signal; an amplitude-demodulator forperforming, in time domain, an amplitude demodulation of saiddown-converted signal in order to generate an envelope; a correlator forcorrelating, in time domain, said envelope with a predeterminedreference pattern in order to detect a signal reference pattern of saidreference symbol in said signal; and means for performing said framesynchronization based on the detection of said signal reference pattern.33. The apparatus according to claim 32, further comprising means forperforming a fast automatic gain control of said received down-convertedsignal preceding said amplitude-demodulator.
 34. The apparatus accordingto claim 32, wherein said amplitude-demodulator comprises means forcalculating an amplitude of said signal using the alpha_(max+)beta_(min−) method.
 35. The apparatus according to claim 32, furthercomprising means for sampling respective amplitudes of said receiveddown-converted signal, wherein said amplitude-demodulator comprisesmeans for comparing said sampled amplitudes with a predeterminedthreshold in order to generate a bit sequence.
 36. The apparatusaccording to claim 35, wherein said means for sampling comprises meansfor over-sampling said received down-converted signal.
 37. The apparatusaccording to claim 32, further comprising means for applying a result ofthe frame synchronization for a frame in said signal to at least onesubsequent frame in said signal.
 38. The apparatus according to claim32, further comprising means for detecting a location of said signalreference pattern based on an occurrence of a maximum of a correlationsignal output of said correlator.
 39. The apparatus according to claim38, further comprising means for weighting a plurality of maxima of saidcorrelation signal such that a maximum occurring first is weightedstronger than any subsequently occurring maximum; and means fordetecting said location of said signal reference pattern based on thegreatest one of said weighted maxima.
 40. The apparatus according toclaim 39, further comprising means for disabling said means forperforming said frame synchronization for a predetermined period of timeafter having switched-on a receiver comprising said apparatus for framesynchronization.
 41. An apparatus for frame synchronization of amulti-carrier modulated signal having a frame structure, each frame ofsaid frame structure comprising at least one useful symbol, a guardinterval associated to said at least one useful symbol and a referencesymbol, said reference symbol comprising a real part and an imaginarypart, said real part and said imaginary part being equal and beingformed by an amplitude modulated bit sequence, said apparatuscomprising: a receiver for receiving said multi-carrier modulatedsignal; a down-converter for down-converting said received multi-carriermodulated signal; an amplitude-demodulator for performing, in the timedomain, an amplitude-demodulation of said down-converted multi-carriermodulated signal in order to generate an envelope; a correlator forcorrelating, in the time domain, said envelope with a predeterminedreference pattern in order to detect a signal reference pattern of saidreference symbol in said multi-carrier modulated signal; means forperforming said frame synchronization based on the detection of saidsignal reference pattern; means for extracting said reference symbol andsaid at least one guard interval from said down-converted receivedmulti-carrier modulated signal based on said frame synchronization inorder to generate said at least one useful symbol; means for performinga Fourier transform in order to provide a sequence of spectra from saidat least one useful symbol; and means for de-mapping said sequence ofspectra in order to provide a bitstream.
 42. The apparatus according toclaim 41, further comprising means for performing a fast automatic gaincontrol of said received down-converted multi-carrier modulated signalpreceding said amplitude-demodulator.
 43. The apparatus according toclaim 41, wherein said amplitude-demodulator comprises means forcalculating an amplitude of said multi-carrier modulated signal usingthe alpha_(max+) beta_(min−) method.
 44. The apparatus according toclaim 41, further comprising means for sampling respective amplitudes ofsaid received down-converted multi-carrier modulated signal, whereinsaid amplitude-demodulator comprises means for comparing said sampledamplitudes with a predetermined threshold in order to generate a bitsequence.
 45. The apparatus according to claim 44, wherein said meansfor sampling comprises means for over-sampling said receiveddown-converted multi-carrier modulated signal.
 46. The apparatusaccording to claim 41, further comprising means for applying a result ofthe frame synchronization for a frame in said multi-carrier modulatedsignal to at least one subsequent frame in said multi-carrier modulatedsignal.